Aromatic hydrocarbons, Alkanols, Alkanals and alkanones | Jamb Chemistry

Aromatic Hydrocarbons: Structure of Benzene

1. Aromatic hydrocarbons contain one or more benzene rings in their structure.
2. Benzene (C₆H₆) is the simplest aromatic hydrocarbon.
3. Benzene has a hexagonal ring structure consisting of six carbon atoms.
4. Each carbon in benzene is bonded to one hydrogen atom.
5. Benzene exhibits delocalized π-electrons over the carbon ring.
6. This delocalization gives benzene its stability and unique properties.
7. Benzene’s structure is represented as a resonance hybrid of two equivalent Kekulé structures.
8. The bond length between carbon atoms in benzene is intermediate between single and double bonds (0.139 nm).
9. Benzene is planar with a bond angle of 120°.
10. It follows the rules of aromaticity, obeying Hückel's rule (4n + 2 π electrons).
11. Benzene undergoes substitution reactions rather than addition, preserving its aromaticity.
12. Examples of aromatic compounds include toluene, phenol, and aniline.
13. Aromatic hydrocarbons are also called arenes.
14. Benzene is nonpolar and immiscible with water but dissolves in organic solvents.
15. Its molecular formula is C₆H₆, but its actual structure shows delocalization of electrons.
16. Benzene burns with a sooty flame due to its high carbon content.
17. Aromatic compounds can be derived from petroleum and coal tar.
18. Substitution of hydrogen atoms in benzene gives derivatives like nitrobenzene, chlorobenzene, and benzaldehyde.
19. Benzene is a starting material for dyes, drugs, and plastics.
20. The unique structure of benzene makes it a core molecule in organic chemistry.
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Properties and Uses of Aromatic Hydrocarbons

21. Aromatic hydrocarbons are stable and less reactive due to resonance.
22. They have a characteristic aromatic smell.
23. They are immiscible in water but soluble in organic solvents.
24. Aromatic hydrocarbons burn with a sooty flame.
25. Benzene is carcinogenic and toxic when inhaled or ingested.
26. Toluene (methylbenzene) is used as an industrial solvent.
27. Aromatic hydrocarbons are used in the manufacture of plastics, dyes, and pharmaceuticals.
28. Benzene is a precursor for styrene (used in polystyrene plastics).
29. Naphthalene (C₁₀H₈) is used in mothballs as a fumigant.
30. Xylene is used as a solvent in paints and coatings.
31. Aromatic compounds form explosives like TNT (trinitrotoluene).
32. Phenol (a benzene derivative) is used in the production of disinfectants.
33. Aniline, derived from benzene, is used in the manufacture of dyes.
34. Benzene is used in the production of detergents and rubbers.
35. Aromatics like benzene and toluene are key in the petrochemical industry.
    
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Alkanols (Alcohols): General Information

36. Alkanols are organic compounds containing the hydroxyl group (-OH).
37. Their general formula is CnH2n+1OH.
38. Alkanols are classified as primary, secondary, or tertiary.
39. The classification depends on the number of carbon atoms attached to the carbon bearing the -OH group.
40. Primary alcohols: The -OH group is attached to a carbon bonded to only one other carbon (e.g., ethanol).
41. Secondary alcohols: The -OH group is attached to a carbon bonded to two carbons (e.g., propan-2-ol).
42. Tertiary alcohols: The -OH group is attached to a carbon bonded to three carbons (e.g., tert-butanol).
43. Alkanols are polar and soluble in water due to hydrogen bonding.
44. As the carbon chain increases, solubility in water decreases.
45. Alkanols are used as fuels, solvents, and in pharmaceuticals.
    
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Production of Ethanol: Fermentation and Petroleum By-products

46. Ethanol can be produced by fermentation of sugars.
47. Fermentation is the breakdown of glucose by yeast in the absence of oxygen:
    • C6H12O6 → 2C2H5OH + 2CO2.
48. Yeast enzymes (zymase) convert sugar into ethanol and carbon dioxide.
49. The process occurs under anaerobic conditions.
50. Fermentation stops when the ethanol concentration reaches about 12-15%.
51. Distillation is used to increase ethanol purity.
52. Ethanol can also be produced from petroleum by-products via hydration of ethene:
    • C2H4 + H2O → C2H5OH (with phosphoric acid as a catalyst).
53. The fermentation method is common in rural areas.
54. Local examples include the production of palm wine, gin, and local spirits.
55. Palm wine ferments naturally due to yeast present in the sap.
56. Distillation purifies the ethanol obtained from fermentation.
57. Fermentation is a renewable source of ethanol.
58. Petroleum-based ethanol is faster to produce but depends on fossil fuels.
59. Ethanol production supports agriculture and rural economies.
60. Glycerol (C3H8O3), a polyhydric alkanol, is also a by-product of fermentation.
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Reactions of the OH Group and Oxidation

61. Alkanols undergo combustion to form CO2 and water.
62. They react with sodium to form alkoxides:
    • 2C2H5OH + 2Na → 2C2H5ONa + H2.
63. Alcohols react with carboxylic acids to form esters (esterification).
64. Alcohols can be oxidized to form different products:
    • Primary → Aldehydes → Carboxylic acids.
    • Secondary → Ketones.
    • Tertiary → No oxidation under normal conditions.
65. Oxidation is a distinguishing test among primary, secondary, and tertiary alkanols.
66. Acidified potassium dichromate (K2Cr2O7) changes from orange to green during oxidation.
67. Primary alcohols oxidize to aldehydes and then to acids.
68. Secondary alcohols oxidize to ketones.
69. Tertiary alcohols resist oxidation due to the absence of a hydrogen atom.
70. This test helps identify the class of alcohols.
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Alkanals and Alkanones

71. Alkanals are aldehydes containing the -CHO group.
72. Their general formula is CnH2nO.
73. Alkanones are ketones containing the C=O group within the chain.
74. Their general formula is also CnH2nO.
75. Aldehydes are formed by oxidation of primary alcohols.
76. Ketones are formed by oxidation of secondary alcohols.
77. Aldehydes reduce Fehling's solution to form a red precipitate.
78. Ketones do not react with Fehling's solution.
79. Tollen's reagent gives a silver mirror with aldehydes but not with ketones.
80. This chemical test distinguishes aldehydes and ketones.
    
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Importance of Ethanol as an Alternative Energy Provider

81. Ethanol is a renewable fuel derived from biomass.
82. It burns cleanly, producing CO2 and water.
83. Ethanol reduces greenhouse gas emissions compared to fossil fuels.
84. It is used as a fuel or fuel additive in bioethanol blends.
85. Ethanol production supports sustainable agriculture.
86. It is used as a substitute for gasoline in vehicles.
87. Ethanol is biodegradable and reduces air pollution.
88. Countries like Brazil use ethanol as a major fuel source.
89. Ethanol can be produced locally, reducing energy dependency.
90. It provides an eco-friendly energy alternative for the future.
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Distinguishing Classes of Alkanols

91. Primary, secondary, and tertiary alkanols differ based on their structure.
92. Primary alkanols: -OH attached to 1 carbon (e.g., ethanol).
93. Secondary alkanols: -OH attached to 2 carbons (e.g., propan-2-ol).
94. Tertiary alkanols: -OH attached to 3 carbons (e.g., tert-butanol).
95. Oxidation is used to distinguish between the classes.
96. Primary alkanols form aldehydes and acids.
97. Secondary alkanols form ketones.
98. Tertiary alkanols resist oxidation.
99. Reactions with sodium show similar behavior across all classes.
100. Structural analysis and oxidation tests are reliable for classification.
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Final Summary Points

101. Benzene has a unique, stable aromatic structure.
102. Aromatic hydrocarbons are essential in industrial chemistry.
103. Alkanols contain the functional group (-OH).
104. Ethanol can be produced by fermentation and chemical synthesis.
105. Fermentation is eco-friendly and supports local industries.
106. Glycerol is a polyhydric alkanol with multiple uses.
107. Alcohols react with sodium and oxidizing agents.
108. Oxidation tests help distinguish alcohol classes.
109. Alkanals (aldehydes) and alkanones (ketones) contain carbonyl groups.
110. Aldehydes reduce Fehling’s and Tollen’s reagents.
111. Ethanol is a promising alternative energy source.
112. Its clean combustion reduces environmental harm.
113. Aromatic compounds like benzene are used in pharmaceuticals and plastics.
114. Functional group classification simplifies organic chemistry.
115. Distillation purifies local products like gin.
116. Petroleum-derived ethanol is fast but less sustainable.
117. Carbon compounds are essential to life and energy.
118. Bioethanol reduces reliance on fossil fuels.
119. Aromatic and alkanol chemistry drive major industrial processes.
120. Understanding these concepts enables innovation in energy and materials.

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